The present invention relates to sealing glass compositions and, more particularly, to sealing glass compositions for bonding ceramic or metal surfaces. Still more particularly the invention relates to compositions useful in bonding ceramics to produce satisfactory bond strength even when fired at temperatures less than 350.degree. C., preferably less than 300.degree. C.
Semiconductor devices or chips, also known as "dies," are packaged in hermetically sealed ceramic containers to resist breakage and destruction. Such ceramic packages may be made of alumina, silicon carbide, silicon nitride, or other ceramics. A common packaging material is alumina (Al.sub.2 O.sub.3).
Ceramic packages of the type described are typically sealed by applying the sealing glass composition to the surfaces of the ceramic components where sealing is to take place and then heating the assembly to elevated temperatures to effect bonding. A problem in sealing ceramic packages is that conventional sealing compositions typically require firing at temperatures above 400.degree. C. to achieve bonding of the ceramic components with satisfactory bond strength. For example, lead borate-based sealing glass compositions, such as have been commonly used, require firing at 420.degree.-430.degree. C. to produce the desired bond strength. However, many of the new semiconductor devices are much more sensitive to firing temperature than previous devices and their usefulness will be adversely affected by exposure to elevated temperatures. Dies which are sensitive to firing temperature include larger dies and dies employing fine line geometry. In the latter case, lines of Si-SiO.sub.2 -Si which are spaced one or two microns apart are provided at the die surface. Firing at high temperatures to accomplish bonding strengths tends to destroy the usefulness of these dies due to incipient melting of the line compositions which may result in undesirable contact between the lines.
Efforts have been made to provide sealing glass compositions which do not require firing at the elevated temperatures such as are necessary with glasses based on lead borate systems. For example, in U.S. Pat. No. 5,013,697, dated May 7, 1991, sealing glass compositions for bonding ceramic surfaces were disclosed which are capable of being fired at temperatures less than 400.degree. C. while still producing satisfactory bond strength. However, there is a continuing interest in sealing glass compositions capable of being used at even lower firing temperatures and the present invention provides sealing glass compositions capable of developing satisfactory bond strength even when fired at temperatures below 350.degree. C., for example, below 300.degree. C.